In our universe, there are two general forces that are acting (or have acted) that I'm concerned about right now: the repulsive force making the universe expand, and the attractive force (gravity) making it contract.
The ratio between these two forces is sometimes called omega.
In order for the universe to last very long at all without either compacting back into one mass or exploding all the pieces of it so far away from each other that they might as well have solitary existence as one isolated piece, this ratio has to be either 1/1, or something extremely close to 1/1.

There are already several theories about why omega in our universe is so nicely tuned towards 1/1 that it was able to last this long, but I'm thinking of another one.

I'm thinking that the omega ratio could be considered as evidence for a multiverse. Supposing that everything that could happen does and each fulfilled possibility splits into a different universe, we could apply that concept to the time when omega was being tuned. Thinking this way, there should be a large number of universes right from the start, each one with a slightly different value for omega, the vast majority of them far from 1/1. Naturally, since having someone there to observe it would require a universe with an omega very close to 1/1, we find ourselves in such a universe.

Any thoughts about that speculation?

Chris65536

From my understanding, and I'm aware that someone is going to call me up on this almost immedietly, but there is no "replusive force". Only momentum.

When the Universe began (big explosion some 12+ billion years ago), everything got flung outwards away from the central point. The explosion was that big that in 12 billion years we're still flying away at some ridiculous speed. It's like if you throw a ball in the air: there's no "repulsive force" sending it upwards once you've let go.

As for the idea of a 1/1 ratio, most scientists agree that there isn't enough observable matter in the universe to even begin to slow the expansion down. This is why there's a hunt for "dark matter": super-dense materials that happens to be completly invisible and give off little or no radiation (Occam's razor anyone? Maybe there is no dark matter), which would cause the overall density of the Universe to be sufficient to stop expanding outwards.

EDIT: After having done a little more research (ok: wikipedia), there's also something called "dark energy", which is like dark matter but isn't.

Bikerman

Chris65536 wrote:

From my understanding, and I'm aware that someone is going to call me up on this almost immedietly, but there is no "replusive force". Only momentum.

No, I'm afraid that is incorrect (according to current theory). Something is causing expansion of spacetime to accelerate. If your picture were correct (I'll explain why it isn't in a mo) then obviously things would either be 'moving apart' at a constant rate or, more accurately, because of the effects of gravity, would be slowing down. This simply isn't what is happening. Now, how do we explain the acceleration? Einstein did it by including an extra term in the equations of General Relativity. He called it the 'cosmological constant'. He originally included it to keep space 'steady' and later said it was the greatest mistake of his career. In fact, as it happens, the concept of a cosmological constant appears to work and agrees with observed data, so perhaps he got the answer right for the wrong reasons. There are other explanations of the 'expansive' force. Quintessence is one, 'dark energy' is another. There are differences in the theories, too subtle to go into here, but what they have in common is that they all posit expansion as a property of empty spacetime. Left to itself, spacetime naturally expands. The expansive force is very small - much smaller than gravity - so when there is mass around the expansive force is swamped by the gravity caused by the mass, and expansion does not happen. That is why our local 'group' of galaxies are not 'moving apart' - they are bound together by gravity. Expansion only happens where gravity is almost zero - in deep intergalactic expanses. At the moment we have no definitive data to show which is correct but we do know for sure that expansion is accelerating and that something is causing it.

Quote:

When the Universe began (big explosion some 12+ billion years ago), everything got flung outwards away from the central point. The explosion was that big that in 12 billion years we're still flying away at some ridiculous speed. It's like if you throw a ball in the air: there's no "repulsive force" sending it upwards once you've let go.

This is a common picture and unfortunately wrong. Many people get confused by this model of the BB - that of an explosion in which everything flies apart. The BB wasn't really like that. Consider for a moment. If the BB happened at a point in space then we should be able to point to it and say - that is where it started. It would require space to have already existed for the BB to happen in. In fact space and time were created at the BB - there was no space for it to happen in. Things are not actually flying apart, what is happening is that the space between the galaxies is expanding and this gives the impression that the galaxies are zooming away from each other. Imagine a small lump of dough with raisins scattered throughout. The dough is spacetime and the raisins are the galaxies. Put it in the oven and the dough rises (expands) and the raisins move apart. That is a very simple analogy. There is no explosion - simply rapid expansion. There is no 'point' at which the BB happened - in a real sense we are all inside the BB.

Chris65536

Hmm...

A-level physics aren't going to get me very far in this forum, methinks Can't stop me from trying though. Still, you learn from being corrected.

So if (empty) space-time expands naturally, why would there be an assumption that everything will stop expanding? If the space between galaxies expands, then the gravitational attraction between those galaxies will be weaker, right?

And if the expansion is accelerating, then that implies that the rate of expansion is already greater than the rate of collapse due to gravity (assume galaxies move 1km closer to each other in a given time reference, but the space expands 1.5 km in the same time). So it won't stop, because gravity will have less and less of an effect, and there will be more and more empty space (which is expanding), right?

Have I got it?

Bikerman

That's pretty much it, yes.
Expansion should (according to theory) continue to accelerate since more space means more expansion and as the galaxies recede from each other there will be less gravity to 'go around'.
The local galaxies will, however, remain gravity bound. Andromeda, for example, will eventually 'collide' with the Milky-Way (in several billion years time).

PS -

Quote:

Can't stop me from trying though. Still, you learn from being corrected.

Absolutely correct. I'm still learning everyday. If I know the answer I'll try to help. If I don't then I probably know someone who does

SonLight

As a non-cosmologist, I don't even pretend to understand the latest theories about expansion of the Universe. Let me state a view of what I think is the path of development, and please feel free to correct me or expand upon it if you have more knowledge of current thinking.

Einstein believed the Universe was neither expanding nor contracting. His equations showed him that it would collapse on itself without an adjustment, so he suggested a repulsive force, and added a "cosmological constant" to his equations which provided for the force, but didn't explain why it should exist. When increasing redshifts were found, he accepted the premise of an expanding Universe which might later contract, depending upon the relative strength of the inertia of the expansion and the gravitational pull, and acknowledged there was no reason for arbitrarily complicating the equations.

A simple model of the expansion assumes that it continues at the same rate. If that were the case, we could estimate the rate of expansion today and the inverse would be the time since the Universe was infinitely small. A better model recognizes that expansion would be slowed by gravitation, and derives an estimate of the time/size relationship of the Universe, which I believe gives about two thirds of the age based on the first model.

A common idea is that the Universe may collapse and there are repeated big bang cycles, although scientists generally believe that nothing can be known about conditions prior to the last big bang. Often the big bang is referred to as "the beginning of the Universe". It does appear to be the beginning of the observable Universe for us.

Ideas of past fluctuations in the expansion rate are unclear to me, but apparently there is some evidence of an uneven rate. The idea of current acceleration is also unclear. I don't know what evidence there is for either of these ideas, and hope someone can explain the basics in simple terms. Apparentlly the acceleration would be a correction factor to the balance between inertia and gravitation, and I assume it would adjust Einstein's equations in a similar way to the cosmological constant.

Basically you are correct about Einstein's inclusion of the cosmological constant - he did it to maintain a 'steady state' universe and later called it his greatest mistake. Observations of accelerating expansion, however, have led some physicists to the conclusion that Einstein actually got it right by accident. If the expansive force is indeed a property of empty space (the cosmological constant) then one would expect expansion to continue to accelerate as the galaxies become more distant from each other.

DoctorBeaver

Chris - I notice you didn't mention Inflation.

Between 10^-35 seconds and 10^-33 seconds, the universe expanded by a number of orders of magnitude causing it to be much larger than light had had time to travel. I believe it stopped when the energy of the universe dropped below the level of Grand Unification (10^19GeV?).

It is theorised that the current expansion is the same inflationary mechanism, but at a vastly lesser rate.

I have also seen it theorised that expansion is due to an approaching brane.

dreulet

It's not getting bigger we are all shrinking. Every force has an opposite perception. My question is the next bang that happened next door to our known universe which is suspect because it is equal in all directions to a consistent horizon temperature (impossible) This matter heading toward us from that bang won't be noticed inside our universal horizon till the light gets here and lets just say it happened 13.5 billion years ago so the matter converging on us we don't know about will be even closer than it is when we see it. Now is the universe expanding anymore? Our tides come every day, our planets allign every 24,000 years to the galaxy, every 72 trillion times you dip an oreo into milk it blows up, probability says the Universe is infinite and recreating itself not just here but everywhere in the infinite realms of space. All you big Bangers come over to the positive side and grab a hold of hope and Dump the Big Rip, your scaring my kids!

ocalhoun

dreulet wrote:

It's not getting bigger we are all shrinking.

If it were caused by us shrinking, it would appear that all galaxies are receding at the same rate, and it would also probably appear to be happening much slower and to be slowing down.